Part 1 - Dose Exploration will assess the safety and tolerability of multiple doses of drug at two dose levels (5 x 10^9 and 8 x 10^9) and will enroll prior to Part 2. Three subjects will be recruited per dose level. Enrollment will begin with the 5 x 10^9 dose level, and the decision to enroll to the 8 x 10^9 dose level will follow a comprehensive safety evaluation and a standard 3 + 3 dose escalation study design.